KEEPING on TRACK

As demands for productivity have increased, automation systems with rigorous duty cycles are being designed for longer travels, higher speeds, and rapid accelerations. These systems often require carriers that must operate more smoothly, and with lower inertia than traditional applications. By limiting flexing to a specified bending radius and imparting a gentle rolling action to feed lines in and out as machinery moves, cable carriers reduce the potential for premature wear and fatigue.

Gantry robots are a perfect example of a system with tough dynamic requirements. The high speeds and long travels associated with them, as well as their multiple cables and hoses, often present a challenge. Let us explore the design process by investigating a specific example. Say a gantry robot has a travel of 130 ft, a velocity of 8 fps, and an acceleration of 12 fps2 — typical for this application. In addition, such a robot might have a cable/hose payload of 10 lb/ft. Its carrier system must accommodate all these variables.

Where to begin specification? As in any carrier design, determining the internal cavity of the system is the most appropriate place to start. Cavity width and height should allow for 10% clearance on the OD: A 10% safety factor with cables and a 20% clearance for hoses. After selecting the appropriate size based on these required dimensions, the carrier's construction must be considered. A carrier's self-supporting capability can be determined by consulting load charts, published by carrier manufacturers. These show maximum self-supporting spans based on travel length and payload. The travel of our gantry robot example exceeds the self-supporting capabilities of both plastic and metallic carrier systems. For this reason, in this case more support is required.

Support

There are several methods for supporting carriers in long travels, depending on construction and application variables. While carriers are available in many configurations and materials, the most common non-metallic carriers are molded fiberglass-filled nylon. The plastic provides high strength per unit weight and lower overall weight and inertia. The cable/hose payload and acceleration in our gantry application result in high tow forces, so an engineer might select a carrier molded from this nylon to reduce load.

Modular systems are often available with a variety of cross-bar options, including snap-in plastic bars for easy access, bolted bars for maximum strength, low friction aluminum or poly rollers for reduced cable/hose jacket wear, and custom-machined blocks from aluminum or low friction bearing materials.

Keep ‘em separated

Separators should always be considered to compartmentalize the cavity. They can isolate cables and hoses from each other to prevent twisting, interference, and chafing that can result in corkscrewing and wear. For example, four separators might be designed into every other link to split one cavity into five compartments. Available in vertical and horizontal configurations, separators also keep different media from each other. It is important however, to consider dimensional safety factors when using separation, to prevent overcrowding.

A carrier's bending radius is chosen based on the specified minimum bend radius of the cables and hoses to be carried. It is important to confirm that the resulting curve height of the system will fit into the available space. Using our gantry example, a radius of 10.56 in. might be chosen based on the radius of the cables involved. (Curve height equals the radius times two, plus one link height — in this case, 24.38 in.) But again, note that size limitations can be imposed externally as well.

To specify the required length of the carrier system, engineers must determine how the system will be mounted. Specifically, where in relation to the travel will the carrier's stationary end be located? To illustrate, assume on our gantry that it's located at the center of the travel. Then only half of the 130-ft travel length (plus enough carrier to form a curve) would be required. Why? The carrier would extend 65 ft to one side of the center point, and retract 65 ft to the other side. As it turns out, the stationary end would be offset 10 ft from center, resulting in the following value for total carrier length:

Where

T = Total travel, ft

a = Center allowance, ft

d = Offset distance, ft

l = Curve length

= π x radius

SF = Safety factor, in whole link increments

As stated earlier, our robot's travel exceeds the self-supporting capability of any carrier system, so support is required. The most common method of support used with plastic carrier systems is a guide trough. A guide trough is designed to let a carrier sag and glide on itself while containing it laterally. This evens out wear and prevents the upper section of the carrier from toppling.

In our gantry application, a guide trough would be needed to support the carrier; the system might also benefit from reconfiguring to a lower mounting height. This technique is used when high acceleration forces are present. By lowering the mounting height of the carrier, the degree of sag is reduced, thereby reducing the bending moment the sagging to which the carrier section is subjected.

Another consideration in long travel applications is tow force, which is calculated using mass, acceleration, and gliding friction. To further reduce wear, most plastic carriers are equipped with glide shoes: raised sections on the inside of the link to serve as a wear surface between main link sections. When carriers glide on themselves, guide troughs insure that these glide shoes contact each other for smoothest operation and maximum wear capability. Sometimes glide shoes are integral to links, and sometimes they're modular. Low-friction polymer glide shoes can reduce tow force by 30% for less wear and increased safety.

Noise considerations

If a quieter design is required, special thermoplastic-elastomer bumper systems can prevent links from achieving full lockout as they pass through the radius, resulting in carriers that operate smoothly and with little noise and resonance. Upon bumper installation, links that once clicked become silent.

A newer development in carrier technology is the single-piece, extruded design offering significant cost savings, as associated tooling and assembly costs are a fraction of traditional injection-molded and link-style carrier setup costs. Another benefit to non-link systems is the elimination of the polygon effect, resulting in a carrier with exceptional wear characteristics and a smoother, quieter operation. Typically, these carriers are not recommended for long travels due to their shorter self-supporting spans, propensity for lateral movement, and reduced strength. However, they can be an ideal solution for applications that require a quiet, low-cost carrier.